EVIDENCE FOR THE IMPORTANCE OF GROWTH-SURFACE STRUCTURE TO TRACE-ELEMENT INCORPORATION IN TOPAZ

Trace element distributions in topaz [Al2SiO4(F,OH)2] single crystals display both sectoral and intrasectoral zoning of trace elements as a function of growth-surface structure. Differential interference contra st microscopy shows that the dominant {110} form exhibits large, polyg onized, spiral growth hillocks with four vicinal faces, each comprisin g an array of parallel growth steps. Cathodoluminescence microscopy an d synchrotron X-ray fluorescence microanalysis show that during growth trace elements (As, Fe, and Ti) were differentially incorporated into vicinal faces having symmetrically nonequivalent step orientations, p roducing intrasectoral zoning in the bulk crystal. Subsectors that for med from vicinal faces related by surface symmetry possess identical c ompositions. In all cases, there is precise correlation between surfac e microtopography and trace element distribution, demonstrating a surf ace-structural control of trace element incorporation. Concentrations of As in time-equivalent regions of different subsectors within the {1 10} growth sector differ by as much as 9:1. Ti and Fe also differ sign ificantly. Similar intrasectoral zoning related to surface-growth step orientation occurs on spiral growth hillocks on the {010}, {001}, and {111} forms as well. Sectoral zoning is also present in the majority of samples examined. Differences in trace element concentrations betwe en time-equivalent regions of symmetrically nonequivalent growth secto rs are large, up to 100:1 for As and between 23:1 and 7:5 for Fe, Ti, Ga, Ge, and Nb. Trace elements proxy for major elements in the structu re but possess differences that significantly affect their incorporati on at the surface. Zoning patterns preserve a record of that effect. A rrays of growth steps migrating in nonequivalent directions on a given surface, as well as growth surfaces of nonequivalent crystal forms, d iffer in their detailed structure of incorporation sites and can diffe rentially incorporate trace elements into coeval regions within a sing le crystal. Therefore, no single value of the distribution coefficient of a trace element can be applied for all areas of the crystal surfac e during growth. This indicates that the value of the distribution coe fficient is path-dependent and thus not governed by equilibrium. Moreo ver, nonequivalent growth step orientation exhibit differences in such kinetic properties as step spreading (growth) rate and degree of curv ature.